As LSI circuits are increasing in density, the line width of circuits of semiconductor devices is becoming finer. Examples of methods of producing an exposure mask (also called a reticle that is used in a stepper or a scanner) to be used to form a circuit pattern for such a semiconductor device include an electron beam writing technique with high resolution.
As electron beam writing apparatuses, writing apparatuses using multiple beams have been developed as an alternative to conventional, single beam writing apparatuses that deflect a single beam to irradiate a desired area of a substrate with the beam. Using multiple beams can significantly improve throughput, because more beams than in the case of writing with a single electron beam can be applied. For example, a multi-beam writing apparatus allows an electron beam emitted from an electron gun to pass through an aperture member having a plurality of holes to form multiple beams, each of which is blanking-controlled by a blanking aperture array. Then, beams that have not been blocked are reduced by an optical system and applied to a substrate on a movable stage.
For multi-beam writing, adjustment, such as focusing, of an optical system is important. Conventionally, by scanning a linear reflective mark on a stage to detect reflected electrons while varying the focus position with an objective lens, focusing has been performed on the basis of a profile acquired at each focus position. The reflective mark needs to be scanned orthogonally to the direction in which the linear reflective mark extends. When the focus position is shifted by the objective lens, the shape of the entire multi-beam image on the substrate is rotated. Therefore, conventionally, the rotation of the entire multi-beam image has been cancelled by making the outputs of two objective lenses in conjunction with each other on the basis of a correlation coefficient. However, this requires calculating the correlation coefficient between the two objective lenses in advance.
Multiple beams include many beams (e.g., about 260000 beams). Therefore, it has been extremely difficult to manufacture the aperture member that forms the multiple beams and the blanking aperture array that controls the blanking of each beam without creating any defects at all. Defects of the aperture member or blanking aperture array may cause unintended beams to reach the substrate, or may block desired beams from reaching the substrate. With the conventional technique which involves scanning a reflective mark, such defects may lower the SN ratio.